1. Field of the Invention
This invention relates to gas precharged accumulators for use in hydraulic pressure fluid systems. Such hydraulic fluid systems may be used to power various given devices. For example, high pressure hydraulic fluid systems may be used to retract aircraft landing gear or to start auxiliary power units in aircraft.
2. Statement of the Prior Art
In the prior art there are typically three types of accumulators. One type uses a piston and cylinder design, another type uses a metal bellows design and the other type uses a bladder or flexible diaphragm. The piston design tends to be generally more light weight and has a relatively simple design. In contrast, although the metal bellows design is more complex, it has the advantage of being able to seal the gas within the chamber much better than the piston design. Although the diaphragm or bladder design provides an excellent seal, it does not have sufficient strength to accomodate the same high differential pressures as the other designs. Earlier models of these designs typically used air as the gas medium under pressure. The air preferably had water and corrosives removed in order to improve performance and increase useful life. In high pressure applications (i.e., approximately 3,000 psi or greater) both designs typically use pressurized nitrogen gas. Nitrogen has the advantage over air in that nitrogen does not tend to corrode the material forming the chamber of the accumulator and being inert does not react with the accumulator material. It is for this reason that nitrogen has found wide acceptance in high pressure accumulator applications.
Nitrogen and ordinary air used as a precharge gas ordinarily give satisfactory performance under moderate pressure and moderate temperature applications; however, under high pressure and very low temperature applications, both nitrogen and air lose a significant amount of energy. This reduces the gas volume and pressure available to exert force. These reductions necessarily reduce the power that can be applied to the hydraulic fluid to actuate the device. Consequently, at low temperature and high pressure applications, the accumulator size must be increased to contain more gas in order to provide the required energy to the system. However, even at at a higher temperature and lower pressure than these, nitrogen and air still lose a significant amount of the energy available to power a device. Consequently, the accumulators must be rather large in order to operate at such low temperatures and high pressures thus increasing their weight considerably. The weight and size of such accumulators clearly present significant disadvantages in aircraft applications.